Recently, femtosecond laser systems are emerging as an alternative to manual incisions in the cornea and crystalline lens for different ophthalmic surgeries. Examples of such laser systems are the Intralase FS Laser and IFS Advanced Femtosecond Laser manufactured and sold by Abbott Medical Optics of Abbott Park, Ill. and the LenSx Femtosecond Laser manufactured and sold by LenSx Lasers of Aliso Viejo, Calif. Such lasers make incisions by focusing ultrashort laser pulses to a very fine focus, causing a plasma mediated photodisruption of the tissue at the points of focus. The incision is generated by placing a contiguous series of such pulses in the pattern of the desired incision. The combined effect of the pattern of pulses is cleaving the tissue at the targeted plane. Arbitrarily complex incision patterns can be generated with such lasers. Furthermore, femtosecond lasers are believed to make more accurate and consistent incisions than the incisions formed manually.
The image space F/# of a beam delivery optical system, such as the femtosecond laser systems described previously, is defined as the focal length relative to the aperture of the system (F/#=f/D, wherein f is the focal length of the beam delivery optical system and D is the entrance pupil diameter). The diameter of a laser spot formed by a beam delivery system is directly proportional to the F/# of the system. Therefore, in general, a low F/# beam delivery optical system is desirable in order to obtain a small focal point in the eye, and therefore maximize the spatial peak irradiance at the focal plane. This allows for a reduction of the laser energy necessary to produce photodisruption, resulting in a smaller shock wave and with smaller zone of collateral damage and less heat transferred to the adjacent tissue. Also, due to the small volume of tissue where photodisruption occurs, a low F/# system allows for high precision cuts with complex patterns.
However, low F/# systems are very susceptible to optical aberrations as the laser beam passes through the optics and the transparent tissue of the eye. Such aberrations alter the spatial irradiance distribution at the focal point, reducing the peak irradiance. Furthermore, in a low F/# system, aberrations vary with the position of the focal point within the eye. For example; in general, the deeper into the tissue the beam is focused, and the further off the axis of the focusing optics the beam is focused, the greater the aberrations. The aberrations are also generally increased significantly when the beam must pass from across curved interfaces between two transparent materials of different refractive indices. Since creating incisions in the cornea and lens requires focusing relatively deep into the tissue, focusing considerably off axis and crossing curved interfaces between transparent materials, it is very challenging to design an optical system capable of focusing a beam over a large three-dimensional working space in the eye using a low F/# beam delivery system while maintaining a relatively unaberrated focal point throughout the entire three-dimensional space.
Currently, there are ophthalmic surgery systems specializing in cutting into the cornea using low F/# beam delivery optical systems, such as the Intralase FS system manufactured and sold by Abbott Medical Optics of Abbott Park, Ill., the VisuMax Femtosecond system manufactured and sold by Carl Zeiss Meditech of Dublin, Calif. and the Technolas Femtosecond Workstation manufactured and sold by Technolas Perfect Vision of München, Germany. However, these systems only cover a limited three-dimensional working space. In particular, although a beam focus must be formed off-axis, the depth of the incisions is limited to no more than the depth of the cornea, about 600 μm. Furthermore, in some of these systems it is required to flatten the cornea in order to eliminate aberrations such as coma and astigmatism that result from the laser beam passing through the curved cornea. No current system has fully addressed the challenging problem of generating a sharp, minimally aberrated beam focus to generate incisions over the full diameter of the cornea and full depth of the cornea and crystalline lens. In other words, no current systems, such as the previously mentioned low F/# beam delivery optical systems, cover any specific method to reduce aberrations while covering a large three-dimensional working space in the eye using a low F/# optical system.